Unscrewing C Mount lenses slightly to achieve better close focus

[Joe G Armstrong]

Hello,

I'm curious to know if you have tried to do the above to improve the close focus of your c-mount lenses on your Bolex for specific shots.

If so, can you please describe to me the various tradeoffs involved in doing this? How much of an impact does this technique have in terms of stop loss? And to what extent does doing this compromise your images in terms of overall resolution? Will my images drop to bits from manipulating the back focus in this way?

I imagine the case is that your mileage may vary. But I'd love to hear/see some examples

[Dom Jaeger]

You can make any lens focus closer by moving it away from the film plane - in fact that's how a lot of lenses work anyway, the entire optical block is shifted away as you turn the focus ring towards minimum. (There are lenses that focus by moving some elements independently, but let's ignore those for the moment.)

The distance you need to move a lens to change the focus is determined by the focal length. A short focal length lens only needs to move a very small amount to shift the focal plane closer. A long focal length needs to move much further, which you can see when you focus a telephoto lens - it shifts considerably more. It's why telephoto lenses often don't have very close focus minimums, because the lens would need to be shifted very far from the film plane. 

The other thing to note is that the relationship is not linear.  To focus closer you need to shift the lens exponentially further away. This is why you get focus distance scales that are compressed as you approach infinity, but very spaced out at the minimum end. It's also why people use extension tubes or bellows to focus down to macro.

If you are just unscrewing a C mount, you only have a few mm of adjustment available before the lens falls out, so it won't get you to macro levels. It can be useful for making a wide angle lens focus a little closer though, especially a fixed focus one. 

The problem of course is that once unscrewed you can't really turn the focus ring and there is a danger that the lens might unscrew further and fall out. It can also droop a bit which could impact the focus across the frame. So the best solution is to make a shim out of plastic that fits around the mount thread, so you can screw the lens in securely. Experimenting with different thicknesses will give you an idea of how the focus shifts. You shouldn't notice any image quality degradation.

If you shim a zoom it will lose parfocality, the wide end will be affected more than the long end. 

Light loss is more of an issue with extension tubes, where you seat the lens a fair distance away from the film plane. You can find the formula online, but as a guide you need to shift a lens 0.4 times the focal length to lose one stop. 

 

[Simon Wyss]

Buy yourself a package of (stainless) steel shims of one inch inner diameter. These can be had in thicknesses up to around 0.011". You want 0.004" thickness. Then you pack the number between camera and lens that suits. Always tighten the lens. Older Kern-Paillard lenses feature longer mounting threads precisely for this purpose. You can unscrew a Wollensak Velostigmat or older Raptar about 2 mm (0.078") without losing it.

You wrote Bolex. Depending on the model you focus by the reflex finder or prefocus by the critical finder or via the CADRO/CADIL accessory.

[Johanan Pandone]

If you just need a little bit shim with aluminum tape